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Parents instinctively protect their children from harm. Seen from this perspective, the assault by Auckland mum Nicola-Jane Jenks on a teenage girl who had bullied her daughter for two years is understandable though it was mistaken.

Jenks, according to the case which unfolded in the Auckland District Court this week, had witnessed distressing changes in her daughter as the bullying took its toll.

The girl's hair thinned, she experienced panic attacks and her school attendance record fell. On the day last May when her frightened daughter rang, Jenks snapped and went after the teenager who had made her daughter's life so miserable.

The girl swore at Jenks, who responded by grabbing the teen's hair and striking her with an open hand with sufficient force that caused bruising to the girl's cheek and scalp.

Her impulsive reaction in striking the girl was, as Judge Tony Fitzgerald remarked, a bad decision made under circumstances which would have tested any parent.

"You should have handled it a lot better than you did," the judge told Jenks. To her credit, Jenks, who had no previous convictions, admitted the assault, undertook counselling and offered to be part of a restorative justice programme.

The judge described her remorse as genuine and discharged her without conviction, noting that the consequences of a criminal record, including overseas travel difficulties, would have been out of proportion to the offending.

It was an appropriate conclusion to the incident, which should never have occurred if a course of action to address the bullying been taken much earlier.

The school the girls attended said the behaviour occurred outside its grounds and was unaware of bullying during school time. The principal said the school did not tolerate violence, harassment or bullying, and was committed to the health and safety of their pupils.

Jenks, however, expressed the view that the incident, which she regretted, was a consequence of her daughter being bullied for two years, and that it was connected to the school.

The circumstances of this case appear to fall in a grey area. School responsibility ought to extend beyond the gate when the well-being and performance of students slips. The fact that Jenks' daughter class attendance was affected by what was happening to her should have prompted some response before matters escalated to the assault in May.

The lessons to be drawn is that schools need to acknowledge that they must sometimes respond to behaviour outside their property. Parents need to know that they can raise these matters with schools and anticipate changes. And mums - or dads - should refrain from taking the law into their own hands, however hard that may seem.